This invention relates to permanent magnet alloys and more particularly to Alnico 9 type permanent magnets in which Co content is smaller than that of the conventional Alnico tupe permanent magnet alloys and which further contain three elements, C, S and Nb, together.
Generally so-called Alnico type permanent magnet alloys contain not only such constituting elements as Al, Ni, Co, Cu, and Fe but also many additives to improve magnetic properties. Further, the improvement of magnetic properties has not been made depending on the composition alone but has been made based on many other factors such as directional property of crystal structure, isothermal magnetic treatment, aging, etc.
These factors contribute, of course, to the improvement of residual flux density (Br), coercive force (Hc), and maximum energy product ((BH) max) but a particularly important purpose of the consideration of these factors is to improve the maximum energy product ((BH) max).
Among Alnico type magnets now available, the one that has the greatest maximum energy product ((BH) max) is Alnico 9 type permanent magnets having a maximum energy product ((BH) max) of 9.0 MGOe or more, which is produced by subjecting a high Ti content Alnico 9 type magnet to a unidirectional solidification to achieve columnar crystalization, subjecting it to solution treatment at a high temperature of 1200.degree. C or more, cooling it rapidly at the cooling rate of 3.degree. C/sec. after the solution treatment, keeping it in a magnetic field at a constant temperature below Curie point for five to ten minutes, and then subjecting it to aging.
This Alnico 9 type permanent magnet, typical composition of which is either Al 7.2%, Ni 14.0%, Co 34.0% Cu 4.0%, Ti 5.0% the remainder being Fe, or Al 7.2%, Ni 13.0%, Co 38.0%, Cu 3.0%, Ti 8.0% the remainder being Fe, all of the above-mentioned percentages being percentages by weight, has a columnar crystal structure after having bean subjected to a unidirectional solidification and has a high maximum energy product ((BH) max), more than twice as much as that of Alnico 5 type permanent magnets used in speakers and motors in general, due to a special heat-treatment. This, however, is not yet produced in an industrial mass production scale.
There are two reasons for it. Firstly, the Alnico 9 type permanent magnet is expensive because it requires much amount of Co and therefore its uses are inevitably limited. In order to obtain the magnetic properties of a residual flux density (Br) of 10000 G (Gauss) or more, a coercive force (Hc) of 1350 Oe (Oersted) or more and a maximum energy product ((BH) max) of 9.0 MGOe (M Gauss Oersted) or more, a Co content of more than 34% is indispensable. Studies have been made to lower the Co content below 34% but the decrease of Co content in Alnico 9 type permanent magnets had a very sharp influence on the magnetic properties and drastically lowered the residual flux density (Br), coercive force (Hc) and maximum energy product ((BH) max). Thus it was extremely difficult to obtain the magnetic properties in a magnet which can be produced on a mass production scale.
Secondly, as it contains a high percentage of Ti, it generally has a finely grained structure which is hard to be made into a good columnar structure by subjecting it to a unidirectional solidification even by such a process as zone melting, hot moulding or exothermic moulding. While the addition of such element or elements as S, C, P, Se, Te, etc. is effective in forming a columnar structure, a single addition of one of these elements can not effect sufficient crystal alignment. A multiple addition of elements such as C plus S effects crystal alignment but does not produce the desired magnetic properties in an Alnico 9 type permanent magnet containing less than 34% by weight of Co.
The present invention has been made in view of the above-mentioned problems involved in Alnico type permanent magnets, particularly, Alnico 9 type permanent magnets and has an object of providing improved Alnico 9 type permanent magnets having a low Co content of 28 - 30% and yet having magnetic properties equal to or superior to those obtained by the conventional Alnico 9 type alloys.